In most cases of moving machinery, mechanical joints formed by various structural elements are subjected to cyclic stress. In the greater majority of cases, the cyclic stresses are relatively small and no substantial fatigue or failure results. In some instances, however, the cyclic stresses can be so large, or can be imposed upon the materials for so long, that fatigue will result. This is especially true if the parts are so constructed as to have a notch, thread, or any abrupt change in cross-section. The maximum stress will occur at that location and will be greater than the nominal stress in the part. In fact, it has been found that threads are often the location at which fatigue failure first occurs as a result of the stress concentration factor.
It has been known for sometime that cyclic stress fatigue could be substantially eliminated by pre-stressing the parts to an extent that the pre-stress imposed is greater than any stress which might be imposed by cyclic operation of the machinery. For example, the parts may be threaded together using a torque wrench to insure that the maximum cyclic stress is exceeded by the imposed pre-stress.
In pre-stressing relatively threadable members, there are two major problems which have not been solved by the prior art devices. In nearly all such devices, it has been necessary to provide a shoulder on either or both of the members in order to produce a sufficient reaction force to generate the pre-stress. Of course, if it is feasible to employ such a shoulder--for example, the head of a bolt--imposing a pre-stressing force becomes a rather simple matter.
On the other hand, it is often necessary to join straight, threaded members in a pre-stressed joint without being able to employ a shoulder against which the joint may be pre-stressed. For example, the joined rods may have to be movably installed in a fairly close fit within another structural element. Unfortunately, if no shoulder can be used to impose pre-stress forces, the last threads of the joint can rarely, if ever, be subjected to the entire pre-stress force. Consequently, the cyclic stresses applied in the vicinity of the last thread may equal or exceed any pre-stress, increasing the chances of failure.